Geomagnetic storms lead to large changes in the Earth's upper atmosphere (ionosphere and thermosphere) that can have adverse effects on technological systems, such as GPS positioning and orbits of satellites in low-Earth orbit (200-2000 km). It is now understood that increases in greenhouse gas concentrations result in a decrease in the thermosphere neutral density. This is primarily due to CO2 being a radiative cooler at high altitudes, leading to a reduction in temperatures in the mesosphere and thermosphere, and a contraction of the upper atmosphere. The ionosphere is also impacted by changes in CO2. The present study is focused on understanding how changes in the background state of the upper atmosphere due to increases in CO2 alter the response of the ionosphere and thermosphere to geomagnetic storms. Using a coupled Earth system model that includes an atmospheric component that extends to the ionosphere and thermosphere, the response of the upper atmosphere to a geomagnetic superstorm is simulated for different levels of CO2 concentrations. It is found that increasing levels of CO2 generally result in a weaker response of the ionosphere and thermosphere to geomagnetic storms in absolute terms, while their relative responses enhance at higher CO2 levels.